As is well known, an EVM is an indicator frequently used to measure the quality of a digital-converted signal. As another indicator for measuring the quality of a digital-converted signal, there is a bit error rate (BER). However, an EVM includes information on the phase of a signal as well as the magnitude, and thus is used to measure signal quality more usefully than BER.
Since an EVM is defined as an error between an ideal transmitted signal transmitted by a transmitting end and a signal received by a receiving end, it is necessary to know the ideal transmitted signal in advance so as to measure an EVM. Meanwhile, binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-symbol quadrature amplitude modulation (16 QAM), and 64 QAM are used as modulation schemes in downlink channels of LTE systems. In the case of these modulation schemes, it is possible to easily detect an ideal transmitted signal through a received LTE message or blind detection, and thus an EVM can be readily measured.
According to the LTE standard of the third generation partnership project (3GPP) technical specification (TS) 36.141, an EVM is defined as shown in Equation 1 below.
                    EVM        =                                                            Σ                                  t                  ∈                  T                                            ⁢                              Σ                                  f                  ∈                                      F                    ⁡                                          (                      t                      )                                                                                  ⁢                                                                                                                                    Z                        eq                                            ⁡                                              (                                                  f                          ,                          t                                                )                                                              -                                          I                      ⁡                                              (                                                  f                          ,                          t                                                )                                                                                                              2                                                                    Σ                                  t                  ∈                  T                                            ⁢                              Σ                                  f                  ∈                                      F                    ⁡                                          (                      t                      )                                                                                  ⁢                                                                                      I                    ⁡                                          (                                              f                        ,                        t                                            )                                                                                        2                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1 above, T denotes a symbol set in a subframe, F(t) denotes a subcarrier set in a symbol t, I(f, t) denotes an ideal transmitted signal at a subcarrier f of the symbol t generated by EVM measurement equipment, and Zeq(f, t) denotes a signal received by the EVM measurement equipment.
As can be seen from Equation 1 above, it is necessary to know an ideal signal at the corresponding subcarrier in advance so as to measure an EVM in an LTE system. Among LTE downlink channels, a primary synchronization signal (PSS), a secondary synchronization signal (SSS), a physical broadcast channel (PBCH), a physical control format indicator channel (PCFICH), a physical downlink control channel (PDCCH), and a physical downlink shared channel (PDSCH) have previously determined modulation schemes, and thus it is possible to easily generate ideal signals through blind detection and so on.
However, in the case of a PHICH in an LTE system, several users use the same orthogonal frequency division multiplexing (OFDM) resources due to the PHICH standard. Therefore, the constellation diagram of a received signal does not have a general digital-modulation shape, and it is difficult to determine an ideal transmitted signal. For this reason, it is difficult to measure the EVM of a PHICH.
FIG. 1 is a sequence diagram of a process for generating a PHICH signal in an LTE system. As shown in FIG. 1, encoded PHICH bits are transmitted through a PHICH, and several PHICHs can be transmitted through a resource element (RE) set constituting one PHICH. Here, such a RE set is referred to as one PHICH group. In this way, several PHICHs can be transmitted through the same PHICH group, and thus a method for distinguishing each PHICH from other PHICHs in the same group is necessary. The distinction is made through different orthogonal sequences.
Therefore, resources of a PHICH have a pair of a PHICH group number nPHICHgroup and an orthogonal sequence index nPHICHseq in a group.
Referring back to FIG. 1, an encoded PHICH bit b(i) is repeated three times (3× repetition) to be three bits for error reduction, BPSK-modulated (BPSK Modulation), as shown in Table 1 below, multiplied by an orthogonal sequence (Orthogonal Code n), and then scrambled (Scrambling). Next, the scrambled PHICH symbol data is processed through layer mapping (Layer mapping) and precoding (Precoding). All pieces of PHICH data precoded in this way are summed and transmitted through one PHICH group.
TABLE 1b(i)lQ01/{square root over (2)}1/{square root over (2)}1−1/{square root over (2)} −1/{square root over (2)} 
Meanwhile, a maximum of eight PHICHs can be transmitted through one PHICH group, and respective PHICHs are distinguished from each other by orthogonal sequences. Here, pieces of data of the PHICHs and the phases of the orthogonal sequences may differ from one another. Therefore, the sum of the PHICHs serves to change the phase and the size of an original modulation symbol, and the PHICH group has an uncommon constellation diagram instead of a BPSK constellation diagram.
As described above, in the case of a PHICH, it is difficult to determine an ideal transmitted signal, and thus it is difficult to measure the EVM of the PHICH.
This work was supported by the ICT R&D program of MSIP/IITP, Republic of Korea. [14-911-01-003, Development of software-based measuring equipment for enhancing inspection of radio station]